1. Field of the Invention
The present invention relates to techniques relating to hard films and, more specifically, to a hard film capable of exhibiting excellent lubricity and wear resistance in an aqueous environment. More particularly, the present invention relates to techniques relating to a hard film for improving the lubricity and wear resistance of sliding members lubricated with a water-base lubricant instead of with a lubricating oil.
2. Description of the Related Art
Most current hydraulic components for driving industrial machines are activated by oil as a working fluid. In some cases it is undesirable to use oil as a working fluid from the viewpoint of preventing environmental contamination by the effluent oil, maintaining sanitation in the food industry and preventing hydraulic devices incorporated into machines using fire, such as incinerators, from catching fire. Studies have been made of the possibility of replacing oil as a working fluid with harmless, nonflammable water as an alternative working fluid.
The following problems arise when water is used as a working fluid instead of oil. Since water does not have lubricity corresponding to that of oil, sliding members of metals will cause seizing if the sliding members are used in an aqueous environment in which water serves as a lubricant. Ceramic sliding members and engineering plastic sliding members have been proposed for use in an aqueous environment. However, such sliding members are expensive as compared with metal sliding members and inferior to metal sliding members in machinability and impact resistance. Therefore, ceramic sliding members and engineering plastic sliding members have not been practically used.
Cutting tools coated with a hard film to improve the wear resistance of the cutting tool have been proposed. For example, mentioned in JP-A2002-18606 is “a coated cutting tool (cutting tool A) formed by coating a cutting tool body with one or a plurality of coating layers, wherein at least one of the coating layers is formed of a material having a chemical composition expressed by: (CraSi1−a) (NxB1−x), where 0.7≦a≦1 and 0.4≦x≦1, and is a CrSi film containing a silicon nitride phase and/or boron nitride phase”. Mentioned also in JP-A 2002-18606 is “in the coated cutting tool A, a coated cutting tool (cutting tool B) is characterized in that at least one of the coating layers is a TiAl film containing at least Ti and Al as metallic elements, and at least N as a nonmetallic element”. Mentioned also in JP-A 2002-18606 is “in the cutting tool B, a coated cutting tool (cutting tool C) is characterized in that dT/dC=0.98 to 1.02, where dT and dC are grating constants calculated by using data obtained through the x-ray diffraction of the (111)-planes of the TiAl film and the CrSi film.
Sliding members coated with the foregoing hard film (coating layer) may be used when the oil as the working fluid for the hydraulic motor for the industrial machine is replaced with water. However, the lubricity and wear resistance of the film (coating layer) having the composition mentioned in the reference in an aqueous environment are insufficient and are unable to prevent the seizing of the sliding members.